1. Field of the Invention
The present invention relates to an X-ray fluoroscopic apparatus and, more particularly, to an X-ray fluoroscopic apparatus using a solid-state imaging device as a TV camera.
2. Description of the Related Art
In a known X-ray fluoroscopic apparatus, an X-ray radiated from an X-ray tube and transmitted through an object to be examined is converted into an optical image by an image intensifier. This optical image is picked up by a TV camera, and is converted into an image signal to display a fluoroscopic image on a monitor. The image signal is converted into a digital signal by an A/D converter, and various kinds of image processing are performed, as needed.
In a conventional TV camera, an interlaced scanning system is employed to respectively scan odd- and even-numbered horizontal scanning lines in odd- and even-numbered fields. In recent years, a solid-state imaging device such as a charge-coupled device (CCD) is often used as a TV camera. The type of the solid-state imaging device is divided into a field storage mode and a frame storage mode with regard to a storing mode of signal charges.
In the field storage mode, each pixel stores signal charges during only a field period. Two pixels adjacent to each other in the vertical direction are read as one pixel, and these two pixels are shifted upward or downward by one pixel in the odd- or even-numbered field. More specifically, two horizontal scanning lines of the imaging device are used as one horizontal scanning line of a TV signal in the field storage mode.
In the frame storage mode, each pixel stores signal charges during a frame period. The charges of the pixels on the odd-numbered horizontal scanning lines are read in the odd-numbered fields, and the charges of the pixels of the even-numbered horizontal scanning lines are read in the even-numbered fields. More specifically, each horizontal scanning line of the imaging device is used as the corresponding horizontal scanning line of a TV signal.
As a result, the vertical resolution in the frame storage mode is twice that in the field storage mode. However, a retained image degrades the quality of the image. Therefore, the usual TV camera includes the solid-state imaging device of the field storing mode.
In the X-ray fluoroscopic apparatus, an X-ray pulse is intermittently radiated. For example, one X-ray pulse is radiated in one frame in synchronism with a frame sync. signal. When a fluoroscopic image is obtained by such radiation of an X-ray pulse, there is no chance to retain the image until the next X-ray pulse is radiated and hence the imaging device is driven in the frame storage mode in many cases.
The frame storage mode in the X-ray fluoroscopic apparatus, however, has the following drawbacks. Pixels on the odd-numbered horizontal scanning lines are first read in synchronism with an X-ray radiation trigger pulse. All the charges in these pixels are read. During reading of a signal in the odd-numbered field, signal charges of pixels on the even-numbered horizontal scanning lines undesirably leak into the pixels on the odd-numbered horizontal scanning lines from which all the charges are read. For this reason, an amount of the signal charges subjected to read from the pixels of the even-numbered horizontal scanning lines is smaller than the actual amount due to the above signal leakage. As a result, an image in the even-numbered field is darker than that in the odd-numbered field. Therefore, when the fluoroscopic image read by such interlaced scanning is displayed on a TV monitor, so-called flickering due to a difference in an amount of the signal between the fields occurs, and it is difficult to observe an image.
Although the CCD type solid-state imaging device has been described above, the above description can be applied to other solid-state imaging devices, such as a MOS type.